1. Field of the Invention
The present invention relates to an electric power steering device in which an auxiliary steering force is imparted from an electric motor to a steering unit of a vehicle such as an automobile.
2. Description of the Related Art
An electric power steering device is a device in which an electric motor generates an auxiliary torque in accordance with a magnitude of a steering torque, and the auxiliary torque is transmitted to a steering unit, to thereby reduce a steering effort required by a driver for steering.
In a general electric power steering device, in order to prevent a steering turn of wheels from exceeding a certain range, a rack shaft has rack ends and a rack housing portion for containing the rack shaft has housing ends, with the rack end and housing end on the same side forming a rack end mechanism. When a steering wheel is turned to a predetermined angle to the right or left from a neutral position and an angle of steering turn of the wheels reaches the maximum (hereinafter, this angle of the wheels, as well as angle of the steering wheel or the like that imparts this angle of the wheels, is referred to as “rack end angle”), in either of the right and left rack end mechanisms, the rack end and the housing end are brought into contact with each other, and thus the wheels cannot perform further steering turn in the same direction.
Accordingly, when the steering wheel is turned close to the rack end angle and then a large steering torque is applied to the steering wheel, the steering wheel soon reaches the rack end angle and the wheels cannot perform further steering turn, and thus the driver may input a larger steering torque. In accordance with this larger steering torque, a larger auxiliary steering force is imparted from the electric motor to the steering unit, which may lead to a large impact on the rack end mechanism, or a large overload on rack-and-pinion gears, resulting in large impact noise, damage or deformation of components, or the like. Furthermore, due to a rotational kinetic energy caused by inertia moment of the electric motor, overshoot of the electric motor rotation may occur, which may also lead to a large impact or large overload on components, such as rack gear, resulting in damage or deformation.
In order to solve these problems, for example, Japanese examined patent publication No. 6-4417 discloses a device in which, when a steering angle (amount of operation) of a steering wheel reaches a predetermined angle at a position near the rack end angle, a target current value is reduced as the turning angle of wheels increases, and when the turning angle of wheels reaches the rack end angle, the target current value is set to zero, to thereby prevent the rack end mechanism from suffering a large impact.
However, in the device of Japanese examined patent publication No. 6-4417, when the steering wheel is turned in an opposite direction at a position near the rack end angle, a target current value is limited until the turning angle becomes the predetermined angle or less, and thus a sufficient auxiliary steering force in accordance with the steering torque cannot be supplied to the steering unit. As a result, the steering wheel becomes heavier, a practical rack end angle to which the driver can operate the steering wheel becomes smaller, and the steering wheel cannot be turned to the rack end angle which is mechanically set.
In order to solve this problem, Japanese unexamined patent publication No. 2006-248252 (see paragraphs [0038] and [0047]) discloses a device in which, when the wheels are at a certain turning angle θ1 or more near the rack end angle, and when a rotational speed ωM of the motor is a set value ω1 or more, a gain of a steering torque value T is reduced.
In addition, when the steering angle (amount of operation) is a certain value or more near the rack end angle, and only when the steering wheel is further tuned away from the home position, a feedback gain of an electric motor angular velocity is increased.
However, in the technique disclosed in Japanese unexamined patent publication No. 2006-248252, when the rack end of the rack shaft and the housing end of the housing, on one of the right and left sides, are brought into contact with each other, and the driver turns the steering wheel further away from the home position, the torque sensor detects a steering torque and thus the motor generates an assist torque in a direction of turning further away. As a result, a stronger force is further applied to the rack end and housing end which have been already brought into contact with each other. In addition, when an absolute value of a rotational speed ω of the motor is smaller than a set value |ω1|, or when the rack end is brought into contact at a high rotational speed ω of the motor, the rotation of the electric motor may undergo overshoot due to the kinetic energy by the inertia moment of the electric motor, and thus components, such as rack gear, may still suffer a large impact or overload. Accordingly, even when the steering wheel is further turned away from the home position under a situation where the rack end and the housing end are already brought into contact, in order to prevent a problem in a mechanical normality of the steering unit which may otherwise be caused by this load, it is necessary to reinforce the motor, the deceleration mechanism (worm gear, worm wheel gear), the rack gear, the pinion gear, the bearing, the housing and the like. Moreover, due to the overload on the motor, the motor may be burnt out.
Therefore, it would be desirable to provide an electric power steering device in which the rack-and-pinion gears do not suffer overload (overload is suppressed), the assist is limited at a position near the rack end, and the components are prevented from suffering impact.